1. Field of the Invention
The present invention relates to an image forming apparatus which is applied to a printing unit of a digital copying machine and a facsimile machine, as well as to a digital printer, a plotter, or the like, and forms images onto a recording medium by jumping developer.
2. Description of the Related Art
There have been proposed printing methods for forming visible images onto recording media such as paper by using electric signals output from a computer, a word processor, a facsimile machine, etc.; the methods include, for example, an ink jet method using ink, a heat transfer method of transferring an image with fused ink, a method subliming chemicals coated on recording paper, an electrophotographic method, etc.
In recent years, in order to meet the demands of faster printing, higher printing quality and lower prices of printing, the ink jet method of non-impact image forming method that allows the apparatus to be formed integrally with a print head in a comparatively simplified configuration is increasingly adopted for image forming apparatuses. Since ink, which is a liquid, is used for the ink jet method, the ink is apt to ooze out on paper and a satisfactory image is difficult to obtain. Besides, in the case of color overprinting, mixed colors made by mixing inks cannot be obtained as expected. Therefore, the electrophotographic method with toner is adopted when high quality printing is required for formed images.
In printing with toner, the toner does not ooze out, and hence an image which has a thick color tone and is excellent in visual appeal can be obtained. In addition, in the case where a plurality of colors are mixed for the purpose of color printing, the colors are mixed in the fixing process and satisfactory mixed colors can be obtained. Accordingly, there is proposed a direct printing method with toner, which is a combination simple processes of the ink jet printing method and toner images.
A prior art technology for an image forming apparatus adopting the direct printing method with toner is disclosed in Japanese Unexamined Patent Publication JP-A 6-218981 (1994). A recording apparatus of JP-A 6-218981 is schematically configured in a manner that a toner holding roller attached to a toner case and a counter electrode are spaced, and between the toner holding roller and the counter electrode, an aperture electrode having a lot of openings is interposed. The electrical field around the respective openings of the aperture electrode is controlled so that a toner flow from the toner holding roller to the counter electrode is controlled. While the toner flow is controlled according to an image to print, recording paper is caused to pass through between the aperture electrode and the counter electrode. Thus, an image is formed onto the recording paper. The recording apparatus further comprises a brush roller for preventing the openings from being clogged with toner stuck on the aperture electrode. Each time an image is printed out, the brush roller comes in contact with the aperture electrode to remove the toner stuck on the aperture electrode.
In the above method that an image is directly formed onto recording paper by jumping such developing particles as toner, the amount of the jumping toner depends on changes of the ambient conditions. For example, when measured at a state where temperature is 20.degree. C. and humidity is 40%, so-called state of normal temperature and normal humidity and at a state where temperature is 35.degree. C. and humidity is 80%, so-called state of high temperature and high humidity, the toner jumping amount is less at the state of high temperature and high humidity.
As means for solving such problems, there has been a proposed method in which a temperature sensor, a humidity sensor, etc. are used to detect and correct ambient conditions and changes of those ambient conditions of the image forming apparatus. In this method, however, the image forming apparatus are provided with sensors and circuits for the sensors, resulting in an increase of the manufacturing cost of the apparatus. In the recording apparatus of JP-A 6-218981, a method for cleaning the aperture electrode in order to remove the toner stuck thereon, and nothing is taken into consideration for controlling the amount of jumping toner according to changes of the ambient conditions.